PDF invoice files stored on the server are named by their invoice number, for example 2023110099.pdf.
PDF files sent by email or downloaded from the Invoice Request page are named with the translation of "Invoice" followed by an underscore and the invoice number, for example Invoice_2023110099.pdf, or Facture_2023110099.pdf, etc.
The Payage payment detail view provides the invoice file name and a link to the file, if it exists.